There is an apprehension that the recent upsurge in the number of users of mobile communication services will cause a shortage of frequencies in the near future. Technicians are looking to an adaptive array method as one solution for this problem.
In the adaptive array method, a directivity pattern is adaptively generated using a plurality of antennas so that radio waves are transmitted only to the user located in a specific direction during transmission and radio waves are received only from the user located in a specific direction during reception. The adaptive array method is described in Adaptive Signal Processing for Spatial Regions and Its Technical Applications (in “Transactions of the Institute of Electronics, Information and Communication Engineers”, Vol. J75-B-II No. 11, November 1992), so that only a brief description is given here. An adaptive array apparatus has a plurality of radio units which each include a transmission circuit, a reception circuit, and an antenna. With this construction, the adaptive array apparatus generates a directivity pattern for transmission and reception by respectively adjusting the amplitude and phase of the transmission signal in each radio unit during transmission and by adjusting the amplitude and phase of the reception signal in each radio unit during reception.
When bidirectional communication is performed using the adaptive array method, it is preferable that devices in communication generate directivity patterns for each other. If the adaptive array method is applied to mobile communication, however, it is impractical for a mobile station to generate directivity patterns due to the physical limitations regarding the scale of the device, the number of antennas, and so on. Therefore, a technique for having a radio base station generate directivity patterns for both reception and transmission has been conceived.
A radio base station employing the adaptive array method can also use a path division multiple access (PDMA) communication method. With the PDMA communication method, radio waves of a plurality of mobile stations are simultaneously multiplexed into one frequency using a plurality of directivity patterns which are respectively generated for the plurality of mobile stations. The PDMA communication method is described in Path Division Multiple Access (PDMA) Mobile Communication Method (in “Shingakugihou”, RSC93-84 (1994–01), pp 37–44) and so is not described in detail here.
When the adaptive array method is applied to mobile communication, a directivity pattern needs to be adjusted according to changes in propagation circumstances with time due to the movement of a user. When analog processing that uses a conventional phase unit is applied to such a mobile communication, various problems concerning the precision, stability, and trackability are caused. Therefore, it is more realistic for the adaptive array method to be realized by digital signal processing.
Digital signal processing is performed to adjust the amplitude and phase of the signal processed in each radio unit. In the case of a digital phase modulation, the amplitude and phase of each symbol is adjusted by appropriately weighting the inphase component and quadrature component of each symbol processed in each radio unit. Accordingly, the signal processing is mainly performed to calculate the weighting factors for the inphase and quadrature components processed in each radio unit. The inphase component and quadrature component of each symbol are hereinafter simply referred to as an I component and a Q component, respectively.
To efficiently use frequencies, it is desired that a radio base station employing the adaptive array method not only generates a plurality of directivity patterns for the same frequency but also avoids interference with nearby radio base stations.
The object of the present invention is therefore to provide a radio base station that efficiently uses frequencies by avoiding interference with the transmission waves of nearby radio base stations.